This invention relates in general to membranes, and more specifically to an electrically neutral nonpermselective porous membrane.
In membrane separation processes there are two forces used to transfer material through the membrane and each is applicable to certain modes of separation. Osmotic and/or hydraulic pressures are used in reverse osmosis, dialysis, simple and ultra-filtration, while an electrical potential is required in electrodialysis. Electrodialysis is unique in itself because the membrane can be designed to selectively transfer either anions or cations, or as in the case of neutral (nonpermselective) membranes, both the anions and cations.
One of the applications of membranes designed for electrodialysis is in the painting of various items, such as auto body parts by electrodeposition. This permits the use of specially formulated water based paints, eliminating the use of solvent and the associated recovery and pollution problems. The water base paint can be designed for either an anionic or a cationic electrodeposition system. This means that the active anionic or cationic portion of the paint is electrically deposited on the item being painted and the oppositely charged free ions generated are removed from the system through a permselective membrane. For the system to work properly, it is important that the undissociated paint molecule touch the item to be painted first which is attached to one electrode of the system. The oppositely charged free ion then generated can move across the system to the oppositely charged electrode where the charge on the free ion is neutralized. The neutralized molecules can now accumulate and would "poison" the system if they were not removed from the painting bath. To prevent the accumulation of inhibiting impurities, the oppositely charged electrode is separated from the bath with an appropriate membrane and the impurity buildup in the chamber between the membrane and the oppositely charged electrode is bled from the system to maintain proper equilibrium.
In cationic electrodeposition, for example, the positively charged paint is deposited on the item attached to the negative electrode. The negative free ion thus generated moves across the painting bath through an anion selective membrane and into the anolyte chamber where the charge is neutralized at the anode and the substance is eventually removed from the system.
It can be seen that an anion selective membrane is used for cationic electrodeposition and that a cation selective membrane is used for anionic electrodeposition. It is possible, however, if the paint is properly formulated and the membrane properly designed, to use a neutral or nonpermselective membrane. This is possible because the unwanted ion, whether it be cationic or anionic, must pass through a membrane into the waste chamber. If the neutral membrane is permeable enough and conductive enough it will allow the transfer of cations or anions. The neutral membrane must also serve as a barrier preventing the flow of the water base paint into the waste chamber. With a properly designed neutral membrane used in an electrodeposition system where the hydrostatic pressure across the membrane can be held to very low levels, all of the required conditions can be met.
It is the purpose of the present invention to design a neutral membrane rugged enough for normal bench work handling in commercial applications and yet durable enough to meet the requirements of a neutral membrane, preferred for use in an electrodeposition system.